Digital cameras photoelectrically convert the image of an object on an image sensor and store the image data in memory or the like. Focus detection devices for automatic focusing of digital still cameras usually rely on one of the following methods: Hill-Climbing focus detection; passive triangulation; active triangulation; phase difference detection; hybrid autofocusing; software focusing correction; and focus detection using a phase control plate and high frequency filters.
In Hill-Climbing focus detection, the taking lens of an image sensing optical system is moved incrementally along the optical axis, and an image is acquired for each incremental lens position. Each of the acquired images are analyzed for focus quality (for example, contrast or sharpness), and after motion through all lens positions and analysis are completed, the lens is moved to the position with best focus for image capture. The lens may be moved to an intermediate position if the software analysis predicts optimum focus at an intermediate plane. US Patent Application Publication No. 2003/0063212 A1 proposes using a Hill-Climbing technique in which the focus evaluation values calculated are weighted in correspondence to the focusing lens position when the focus evaluation values are calculated.
The accuracy of the Hill-Climbing focus detection technique is very good due to the fact that the image itself is used to determine focus quality and hence the final lens position. Many lens motions are however required, making this technique very slow. Another disadvantage is that hill-climbing autofocus systems tend to work poorly in low light situations where contrast levels are generally low.
In passive triangulation focus detection, two images of an object are formed on a focus detection sensor by two optical systems spaced apart by a predetermined base length, and the absolute distance to the object is determined from the spacing between the two images formed. The lens is moved directly to the calculated position for image capture. Passive triangulation is faster than Hill-Climbing focus detection since only one final lens motion is required, however low light situations are still challenging. In addition, since the focal length and base-length of the range-finding optical system cannot be made very large due to space constraints on the camera, to insure focus accuracy, the dimensional precision of the component parts must be very high, and the image forming optical system must be well calibrated with the passive triangulation system
In active triangulation focus detection, a light projection system emits a signal, which is reflected off the subject and subsequently is detected by a light receiving system, such as a silicon photodiode array. The absolute distance to the subject is determined by the spacing between the light source and detected signal. The lens is moved directly to the position for image capture. This technique is very fast since the taking lens only has to be moved once to its final calculated position, and since an active source is used, such as an IR emitter as disclosed in U.S. Pat. No. 6,452,664, low light conditions do not pose a problem. There are two major disadvantages to this method of autofocusing. The first disadvantage relates to accuracy, as with passive triangulation, to insure focus accuracy, the dimensional precision of the component parts must be very high, the component parts such as lenses must be environmentally stable, and the image forming optical system must be well calibrated. In U.S. Pat. No. 6,452,664, a method of using multiple range-finding operations is suggested to improve focus accuracy, however this method does not address the environmental stability of the component parts such as lenses and it does not change the need for close calibration between the image forming system and the passive triangulation system. The second disadvantage to relying on active range-finding is that a faulty lens position could be chosen if there is a transparent obstruction (e.g. glass window pane) between the camera and the object, in which case the lens is focused on the obstruction. Alternatively, if part of the subject is a very smooth surface such as water, and the pulse is incident at an oblique angle, the IR pulse is deflected away from the camera, and focusing cannot be achieved.
In phase difference detection, optical images that have been formed by passage through different pupil areas of an image sensing optical system are formed again as a pair of secondary images via a secondary image forming optical system, and the state of focus is determined from the spacing between the two secondary images. The lens is moved directly to the position for image capture. With phase difference detection since there is no additional light source, transparent obstructions and smooth surfaces are not a problem. In addition, phase difference detection systems are fast since only one final lens motion is required. They have the additional advantage that they can be operated through the objective lens (i.e. by inserting an optical module as disclosed in US Patent Application Publication No. US2001/0045989A1 or by deflecting part of the light to a separate image sensor as disclosed in U.S. Pat. No. 6,643,460B2). The disadvantage of inserting an optical module is that moving parts are necessary. The disadvantage of deflecting part of the light to a separate image sensor is that some of the image forming light is lost. In both cases, calibration is critical since an additional optical path is used to determine focus. An additional disadvantage of this technique is that low light situations can compromise the effectiveness of the technique, requiring active autofocusing as in passive triangulation.
In hybrid autofocusing, a combination of the Hill Climbing focus detection and a range finding technique such as active triangulation is used to reduce disadvantages experienced when either technique is used alone. The range finder provides a rough initial focus quickly, and the Hill-Climbing focus detection system uses a reduced number of images to determine the final lens position with best contrast, see, for example, US Patent Application Publication No. 2004/0100573A1 and U.S. Pat. No. 5,333,028. Hybrid systems are faster than purely Hill-Climbing focus detection based systems since the number of lens motions is reduced, however they are still slower than range-finding alone or phase detection systems, where only one lens motion is required.
In software corrections autofocusing, intelligent processors use scene recognition or predictive lens motion based on calculated image position to infer correct lens position, see, for example, US Patent Application Publication No. 2004/0070679 A1 and U.S. Pat. No. 5,089,843. Image capture occurs continuously and selection of lens position is chosen based on a typical shutter delay. The disadvantage of relying on software correction is that it is prone to errors if a change of velocity or motion direction occurs.
U.S. Pat. No. 5,337,115 discloses focus detection using a phase control plate and high frequency filters. In U.S. Pat. No. 5,337,115, a focus-detecting device is described for a video camera application. A phase plate is mounted onto the image sensor, such that light flux impinging on alternating rows of image sensor elements has traversed one of two possible optical path lengths. The image seen by the first group of elements will thus have a different focus quality than the image seen by the second group of elements. Focus quality in this application is determined by analyzing the high frequency content of the image. From the relative focus quality factors, it is determined whether the image is in focus, and if not, which direction the lens must travel. This technique has the advantage that it is a through the lens technique and it is fast since theoretically only one motion is required. However the image quality is degraded due to the alternating structure of the phase plate (possibly resulting in banding, for example).